1. Field of the Invention
The present invention relates to an emission control apparatus for an internal combustion engine and, more particularly, to an internal combustion engine emission control apparatus having an NOx sensor for detecting an NOx concentration in exhaust gas.
2. Description of the Related Art
An emission control apparatus having an NOx sensor for detecting a concentration of oxides of nitrogen (NOx) in exhaust gas from an internal combustion engine is described in, for example, Japanese Patent Application Laid-Open No. HEI 7-166851. In the emission control apparatus, an NOx-absorbing and reducing catalyst is disposed in an exhaust passage of the engine, and the NOx sensor is disposed in a portion of the exhaust passage downstream of the catalyst so as to detect the NOx concentration in exhaust gas that has passed through the NOx-absorbing and reducing catalyst. The NOx-absorbing and reducing catalyst absorbs NOx from exhaust gas when the air-fuel ratio of exhaust gas flowing into the catalyst is on a lean side of the theoretical air-fuel ratio. The catalyst releases NOx when the oxygen concentration in exhaust gas flowing into the catalyst decreases. Therefore, the NOx-absorbing and reducing catalyst is used as an NOx-lessening catalyst.
The NOx-absorbing capability of NOx-absorbing and reducing catalysts decreases with an increase in the amount of NOx absorbed therein. Therefore, as the amount of NOx absorbed in an NOx-absorbing and reducing catalyst increases, the amount of NOx that passes through the NOx-absorbing and reducing catalyst without being adsorbed therein increases, so that the amount of NOx remaining in the exhaust gas from the exhaust system increases.
Therefore, when the NOx concentration in exhaust gas detected by the NOx sensor disposed in a portion of the exhaust passage downstream of the NOx-absorbing and reducing catalyst increases to a predetermined value, the aforementioned apparatus causes the engine to operate at a fuel-rich air-fuel ratio for a short time so as to change the condition of exhaust gas flowing into the NOx-absorbing and reducing catalyst (in this case, change the exhaust gas air-fuel ratio to a rich side). Due to this operation, NOx is released from the NOx-absorbing and reducing catalyst, so that the NOx-absorbing capability of the NOx-absorbing and reducing catalyst is reactivated.
When the air-fuel ratio of exhaust gas from the engine shifts to the rich side, the oxygen concentration in the exhaust gas decreases and the quantity of components that function as reducers, such as unburned hydrocarbons (HC), carbon monoxide (CO) and the like, increases. When the oxygen concentration in the exhaust gas decreases, NOx is released from the NOx-absorbing and reducing catalyst. That is, the amount of NOx stored in the NOx-absorbing and reducing catalyst decreases. An amount of NOx released from the NOx-absorbing and reducing catalyst reacts with reducer components in exhaust gas on the catalyst, thereby lessening the amount of NOx present in exhaust gas.
However, if the release of NOx from the NOx-absorbing and reducing catalyst and the reduction thereof are controlled on the basis of the exhaust NOx concentration detected by the NOx sensor, a problem may arise if the NOx sensor has deteriorated so that the NOx sensor output drifts. In this case, an NOx concentration indicated by an output of the NOx sensor may deviate from the actual NOx concentration.
If such a sensor output deviation occurs, it becomes difficult to properly perform the release and reduction of NOx (reactivation of the NOx-absorbing and reducing catalyst). Unnecessary performance of the reactivating process may result in degradation of the engine fuel consumption, release of unburned HC and CO into the atmosphere, and an outflow of NOx from the NOx-absorbing and reducing catalyst downstream due to an increased amount of NOx stored in the NOx-absorbing and reducing catalyst, and the like.
Problems substantially the same as those stated above also occur in an apparatus that employs, as an NOx-lessening catalyst, a selective reducing catalyst that is able to selectively reduce NOx present in a lean-air-fuel ratio exhaust gas.
A selective reducing catalyst reduces NOx by using HC and CO components of exhaust gas. If the amounts of HC and CO components present on the selective reducing catalyst become insufficient, the amount of NOx that flows downstream from the catalyst without being reduced increases. For example, in an apparatus in which the amount of NOx that flows out of a selective reducing catalyst without being reduced thereby is detected by an NOx sensor disposed downstream of the catalyst and, in accordance with the amount of unreduced NOx, the engine operating air-fuel ratio is changed to adjust characteristics of exhaust gas (in this case, the amounts of HC and CO components) flowing into the catalyst, a deviation of the output of the NOx sensor causes emission of unreacted HC and CO components into the atmosphere, emission of unreduced NOx into the atmosphere, or other problems.